Circular waveguide mode filter

ABSTRACT

A circular waveguide mode filter is provided which comprises a circular waveguide consisting of an upper and lower semicircular waveguide sections and in which dielectrics or magnetic materials are disposed in either of the upper or lower waveguide sections or in both of them in a special spatial relation with them so that the desired TE01 signal mode may be propagated through the waveguide while the undesired TE0n (where n &gt; OR = 2) modes are absorbed.

United States Patent [191 Shimada et a1.

[ CIRCULAR WAVEGUIDE MODE FILTER [75] Inventors: Sadakuni Shimada; KunioHashimoto; Ken Kondoh, all of Tokyo; Masaki Koyama, Sayama, all of Japan[73] Assignee: Nippon Telegraph and Telephone Puhlic Corporation, Tokyo,Japan [22] Filed: July 14, 1972 [21] Appl. No.: 272,037

[30] Foreign Application Priority Data July 19, l97l Japan 46-53688 [52]U.S. Cl. 333/98 M, 333/21 R, 333/31 A [51] Int. Cl. 1101p 1/16, HO1pl/l8[58] Field of Search.... 333/98 M, 95 R, 21 R, 31 A [56] ReferencesCited UNlTED STATES PATENTS 2,l29,669 9/1938 Hnwcn 333/21 R Jan. 22,1974 2,762,982 9/1956 Morgan, Jr l. 333/21 R 2,951,219 8/1960 -Marcatili333/21 R 3,321,720 5/1967 Shimada 333/21 R Primary Examiner-EliLieberman Assistant Examiner-Wm. H. Punter [57] ABSTRACT A circularwaveguide mode filter is provided which comprises a circular waveguideconsisting of an upper 10 Claims, 19 Drawing Figures PMEMEMNZZ'QM Y3.787.787"

SHEET 1 F '5 Q5462R 0.5462R THE ELECTRIC FIELD THE ELECTRIC FIELDDISTRIBUTION OF THE DISTRIBUTION OF THE TEol MODE TEO2 MODEPATENIEDJANZEIQH 5.787.781

SHEET 2 F FIG. 3(0) FlG. 3(d) THE MAGNETIC FIELD THE MAGNETIC FIELDDISTRIBUTION OF THE DISTRIBUTION OF THE TEO| MODE TEO2 MODE 2 T I FIG.4(0) 5 THEORETICAL VALUE 8 EXPERIMENTALVALUE WLMLE I.

$ 40 50 so 70 so Z FREQUENCY (GHZI INSERTION LOSS CHARACTERISTIC FORTEO| MODE (EXCLUDING DIELECTRIC) Q 40 FIG. 4(b) v THEORETICAL VALUE (f)8 3o EXPER|MENTAL VALUE I g E 0: IO; a \7 3 0 so so FREQUENCY (GI-IZIABSORPTION LOSS CHARACTERISTIC FOR TEO2 MODE (EXCLUDING DIELECTRICIPAIENIEIIJIII22IIITI 3' 787 787 SHEET (If 5 FIG. 5(6)v I IO PRESENTINvENTIoN 5 WAVE-COUPLED TYPE I I (D 2 LLI 0.5 J

| I .O'IIO I5 20 25 3 35 RADIUS (mm) RELATION BETWEEN RADIUS RI I AND 1I bLg-INGTH (m) OF TEO2 MODE FILTERS 3o I I} LLI 85520 '3 z i INNERDIAMETER sImm PREsENT-INvENTloN I WAVE-COUPLED TYPE 0 I I I I I -o.3 -o2-o.I 0 0| 0.2 0.3

DIMMENSIONAI. ERRORS AR (mm) EFFECT UPON MAXIMUM ATTENUATION OF wgIsIoNAL ERRoRs OFRAD'IUS OF WAVE PMEMEMMH 5.187. 787

SHEET 5 UP 5 FIG. 7(6) FIG Nb) FBG. 9(b) .waveguide mode filter lCIRCULAR WAVEGUIDE MODE FlLTER BACKGROUND OF THE INVENTION The presentinvention relates to generally a filter used in TE mode transmissionlines composed of circular waveguides and more particularly a circularcapable of giving high attenuation to the undesired higher modes such asthe circular TE mode without affecting the propagation of the desiredcircular TE mode.

Circular waveguides used in the millimeter wave communication systemsgenerally have an inner diameter considerably greater than thewavelength of the desired TE mode in order to reduce the attenuation dueto the wall heat loss of said mode. For example, the circular waveguidefor a millimeter wave communication system whose frequency range is from40 to 80 GHz or from 40 to l20 GHz has an inner diameter from 40 to 60mm. Therefore, a considerable number of modes can be propagated throughthe circular waveguide. When the helix waveguides are used as thetransmission lines, the undesired modes other than the circularlysymmetric modes such as TE TE and TE may be sufficiently suppressed, sothat the circular waveguide line must be generally provided with modefilters'for attenuating the undesired TE modes (n E 2). Furthermore, theundesired modes TE (n 5 2) are generated in the corner waveguides whichmay be regarded as a sort of mirror capable of bending the microwaves atsharp angles at bends. ln inter-city or interoffice trunk lines whichhave a considerable number of bends, the corner waveguides are used sothat the degradation of the transmission characteristic occurs due tothe conversion and reconversion of the TB. signal mode and the undesiredTE (n y; 2) modes. Therefore, mode filters must be provided capable ofabsorbing the undesired modes generated in the corner waveguides.

ln general, of the undesired modes, the lowest mode, TE mode is mostdominant so that when the latter is sufficiently suppressed or absorbed,other undesired modes will not present a serious problem inthe'microwave transmission ,lines in practice. Therefore, the presentinvention is mainly directed to the absorption and attenuation of the TEmode, but it should be understood that the present invention may be alsoapplied to the absorption and atteniiation of other undesired modes.

ln order to absorb and attenuate the undesired TE mode, there has beenproposed a wave coupling type mode filter in which a large number ofcoupling holes are formed through the wall of an inner circularwaveguide so that of the TE and TE modes propagating through the innerwaveguide, the undesired TE mode may be directed into an outer circularwaveguide through the coupling holes. When the wave coupling type modefilter of the typedescribed above is inserted into the microwavetransmission line in practice, tapered waveguides with a considerablelength must be coupled to both ends of the wave coupling type modefilter so that various problems arise such as over-all length of thewave coupling type mode filter including the tapered waveguidecouplings, of the confined resonance, the dimensional tolerances orerrors caused in manufacturing and laying, and the like.

material having a large number of slots formed at a point where thefield intensity of the TE signal mode becomes zero is inserted in thewaveguide so as to divide the waveguide into two sections. Therefore,when TE and TE modes propagate through the waveguide, the undesired TEmode may be absorbed by the lossy material. However, the attenuation ofthe undesired TE mode higher than 2 dB/m attained by the resonant slottype mode filter is in the relatively narrow frequency band of 5 Gl-lzat 50 Gl-lz. In other words, the resonant slot type mode filter is notadapted for use in the broad band. Furthermore, the higher thefrequency, the lower becomes the attenuation of the undesired TE mode.

Furthermore, there has been devised and demonstrated a circularwaveguide mode filter of the type comprising a pair of upper and lowersemicircular waveguide sections having different radii. Theunderlyin'gprinciple of the circular waveguide mode filter is based onthe fact that the phase velocities of the semicircular modes propagatingthrough the semicircular waveguide sections having different radii aredifferent from each other. Based upon this principle, the undesired TEmode maybe attenuated without the desired TE mode being adverselyaffected. More particularly, for the desired semicircular TE mode, thephase'difference between the pair of semicircular waveguide sections isan integer integral of 2w, (that is 2mr'where n 0, l, 2, 3 so that theelectric fields of the TE. modes may be directed in the same directionsat the outlet of the each semicircular waveguide. As for the undesiredTE mode, the phase difference of the TE modes propagating through theupper and lower waveguide sections is made an odd integer of 11', (thatis, (2n +1)rr where n=0, 1, 2, 3, 4, so that the electric fields at theoutlet of the waveguide are directed in opposite directions. Therefore,the two semicircular TE modes propagating through the upper and lowersemicircular waveguides are composed into one circular TE mode and arenot affected at all, whereas the two semicircular TE modes at the outletof the waveguide are converted into the TM mode and the like, which maybe absorbed by the helix waveguide. However, in the circular waveguidemode filter of the type described, the specific phase relationsdescribed above are not maintained when the frequency varies sothat theinsertion loss of the TB mode is increased. As a result, the prior artcircular waveguide mode filter is not effective over a broad band.Furthermore, the attenuation of the undesired TE mode is dependentsolely upon the mode conversion at the output of the waveguide so thatthe multiple reflections in the semicircular waveguide sections occur.As a result, the TE mode absorption effect is reduced and the reflectioncharacteristic of the desired TE, mode is adversely affected.

At the outlet of the waveguide, the mode conversion generates diversemodes some of which cannot be effectively absorbed by the helixwaveguide. Therefore, the reconversion into the TE mode will occur at.the imperfect portions of the helix waveguide.

One of the objects of the present invention is therefore to provide animproved circular waveguide mode filter with a broad band width.

Another object of the present invention is to provide an improvedcircular waveguide mode filter which is shorter in over-all length,simple in construction and reliable and dependable in operation and notso severe in manufacturing and laying tolerances or errors as comparedwith the prior art mode filters.

According to one aspect of the present invention, a circular waveguidemode filter comprises a pair of semi-circular waveguide sections havingdifferent radii, and a dielectric disposed in the semicircular waveguidesection having a smaller radius at such a position where said dielectricwill not adversely affect the TE mode propagating through saidsemicircular waveguide section with a smaller radius. The phase velocityof the TE mode propagating through the semicircularwaveguide sectionwith a smaller radius becomes different from that of the TE modepropagating through the other semicircular waveguide section with agreater radius so that the field intensity of the TE mode propagatingthrough the semicircular waveguide section with a smaller radius isdirected at the outlet of the mode filter in the direction opposite tothat of the field intensity of the TE mode propagating through the othersemicircular waveguide section with a greater radius. As for the desiredTE mode, the apparatus radius of the semicircular waveguide section witha smaller radius becomes equal to that of the other semicircularwaveguide section with a greater radius because of the dielectric ormagnetic material disposed in the semicircular waveguide section with asmall diameter so that the TE modes propagating through the bothwaveguide sections become equal in phase velocity. Thus only thecircular TE mode is derived from the outlet of the circular waveguidemode filter whereas the undesired circular TE mode is absorbed by themode filter.

The above and other objects, features and advantages of the presentinvention will become more apparent from the following description ofthe preferred embodiments thereof taken in conjunction with theaccompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. Us a perspective view partly insection of a circular waveguide mode filter in accordance with thepresent invention;

FIG. 2 is a cross sectional view thereof;

FIGS. 3a through 3d illustrate the electric and magnetic' fielddistributions of the circular TE and TE modes used for explanation ofthe underlying principle of the present invention;

FIGS. 4(a) and 4(b) are curves ilustrating the insertion loss of the TEmode and attenuation loss of the TE moade respectively of a circularwaveguide without the dielectric of FIG. 1;

FIGS. 4(c) and 4(d) are curves corresponding to FIGS. 4(a) and 4(b)respectively when a dielectric is employed, as shown in FIG. 1.

FIGS. 5a and 5b illustrate the advantages of the circular waveguide modefilter in accordance with the present invention over the prior art wavecoupling type mode filter;

FIGS. 6(a) and 6(b) are cross sectional views of a second and a thirdembodiments of the present invention of the type utilizing thedielectric materials;

FIGS. 7(a) and 7(b) are cross sectional views of a fourth and fifthembodiments of the present invention of the type utilizing the magneticmaterials;

FIG. 8 is a longitudinal sectional view of a modification of theembodiment shown in FIG. 1; and

FIGS. 9(a) and 9(b) are top views of modifications of a thin metallicslab or sheet partition wall having resistor elements or films.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a circularwaveguide mode filter generally designated by l in accordance with thepresent invention generally comprises an upper and lower semicircularwaveguide sections 10 and 20 partitioned from each other by a thinmetallic slab 2. The middle waveguide section 13 of the uppersemicircular waveguide section 10 has a radius smaller than that of thelower semicircular waveguide section 20, and a semicylindricaldielectric 3 having a radius smaller than that of the middle waveguidesection 13 is disposed inside the section 13 coaxially thereof. Moreparticularly, as shown in FIG. 2, the radius R of the lower waveguidesection 20 is greater than the radius R of the middle waveguide section13, and the semi-cylindrical dielectric 3 has a radius equal to0.5462R'. The middle waveguide section 13 has both of its ends joined tothe semicircular waveguide sections 11 and 15 on the sides of the inputand output respectively, through tapered semicircular waveguide sections12 and 14, respectively.

In a semicircular or circular waveguide, the radial field distributionsof the transverse electric field components E and the longitudinalmagnetic field components Hz of the TE and TE modes are shown in FIG. 3.That is, the electric field distributions of the TE and TE modes areshown in FIGS. 3(a) and 3( b), respectively, whereas the magnetic fielddistributions are shown in FIGS. 3(c) and 3(d), respectively. Forexample, in FIG. 3(b), the electric field component E of the TE modebecomes zero at a point spaced apart by 0.5462R from the center.Therefore when the dielectric 3 is disposed as shown in FIG. 2 at adistance of 0.5462R from the center of the upper semicircular waveguidesection 10, the TE semicircular waveguide propagating through thesection 10 is not substantially affected by the dielectric 3 except thatits phase velocity is changed because the radius of the middle waveguidesection 13 is smaller. As for the TE mode propagating through thesemicircular waveguide section 10, the apparent radius of the section 10becomes greater because of the influence from the dielectric 3. The

. phase velocities of TE modes propagating through the semicircularwaveguide sections 10 and 20 are different from each other because ofthe difference in radius of the upper and lower semicircular waveguidesections 10 and 20. Therefore, it becomes possible to reverse thedirections of the TE modes propagating through the waveguide sections 10and 20 at the outlets thereof if the lengths thereof are suitablyselected. Furthermore, for the TE modes the apparent radius of the uppersemicircular waveguide 10 may become equal to the radius of the lowersemicircular waveguide section 20 when the radii of the upper and lowersemicircular waveguide sections 10 and 20 and the dielectric material 3are suitably selected. As a result, there will be difference in phasevelocity between the TF modes propagating through the upper and lowersemicircular waveguide sections 10 and 20. That is, the directions ofthe electric fields of the TE., modes at the outlets of the semicircularwaveguide sections 10 and 20 become equal. In this case, the TE modespropagating through the upper and lower semicircular waveguide sections10 and 20 have no phase difference.

In the experiments conductd by the inventor, the circular waveguideshown in FIG. 1 comprising the input and output waveguide sections 11and 15, 100 mm in length, the tapered waveguide sections 12 and I4, 75mm in length and the middle section 13, 300 mm in length and having theoverall length of 650 mm was used. The radius R of the uppersemicircularwaveguide section 10 was 23.0 mm whereas the radius R of thelower semicircular waveguide section 20 was 25.5 mm. Thesemi-cylindrical dielectric 13 with a dielectric constant of 1.03 and alength of 100 mm and a thickness of 2.0 mm was disposed in thesemicircular waveguide section 10 at a distance of 0.5462R' from thecenter thereof.

The experimental results are shown in FIG. 4. FIGS. 4(a) and 4(b) showthe insertion loss characteristic of the TE mode and attenuation losscharacteristic of the TE mode when the dielectric 3 was not used. Asseen from FIG. 4(a), when the dielectric 3 is not used, the loss of theTE mode signal is relatively higher, about 2 dB at a low frequency, andtherefore is not satisfactory in practice. However the better TE modecharacteristic corresponding to the theoretical value may be obtained.FIGS. 4(c) and 4(d) show the corresponding characteristics when thedielectric 3 was inserted. In FIG. 4, the experimental values areindicated by the solid lines whereas the theoretical values, by thebroken lines. From FIG. 4, it is readily seen that the improvement overthe loss characteristic of the TE mode can be attained by the insertionof the dielectric 3 into the semicircular waveguide section 10 with thesmaller radius of the circular waveguide l. The reason is that, asdescribed above, the apparent radius of the upper semicircular waveguidesection 10 for the TE mode becomes equal to the radius of the lowersemicircular waveguide section 20. The effect of the dielectric 3 uponthe TE mode is almost negligible, and the ab sorption loss becomesgreater because of the difference in radius between the upper and lowersemicircular waveguide sections 10 and 2.

The advantages of the mode filter in accordance with the presentinvention over the prior art wave-coupled type mode filter are bestshown in FIG. 5. FIG. 5(a) shows the relation between the radius andlength of the filter; and FIG. 5(b), the relation betwen the dimensionalerror in radius of the filter and the maximum attenuation of the TEmode. In FIG. 5, the mode filter in accordance with the presentinvention is designated by the solide lines, where as the prior art modefilter, by the broken lines. It is readily seen that the overall lengthof the mode filter in accordance with the present invention is shorterthan that of the prior art mode filter and that the dimensional errorsor tolerances required for the mode filter in accordance with thepresent invention are not so severe as those required for the prior artmode filter.

In addition to the mode filter for the circular waveguide described sofar with reference to FIGS. 1 and 2, various modifications andvariations can be effected.

In the second embodiment of the present invention shown in FIG. 6(a),the circularwaveguide comprises the upper and lower semicircularwaveguide sections 10 and partitioned from each other by the metallicslab or sheet 2 as in the case of the first embodiment, but instead ofthe semi-cylindrical dielectric 3, dielectric rods 3 and 3' aredisposedin the upper semicircular waveguide section 10 with a smallerradius at a distance equal 0.54-62R respectively on both sides of thecenter of the section 10. The mode of operation of the second embodimentis substantially similarto that of the first embodiment. That is, theeffects of the dielectric rods 3 and 3' upon the TE mode propagatingthrough the semicircular waveguide section 10 are almost negligible, butthe phase velocity is changed as the radius of the upper semicircularwaveguide section 10 is smaller. As for the TE mode, the apparent radiusof the upper semicircular waveguide section 10 becomes greater becauseof the presence of the dielectric rods 3 and 3 so that the phasevelocity of the TE mode propagating through the upper semicircularwaveguide section 10 equals that of the TE mode propagating through thelower semicircular waveguide section 20.

In the third embodiment shown in FIG. 6(b), the upper, and lowersemicircular waveguide sections 10 and 20 have the same radius, andsemicircular dielectric 3 is disposed within the upper semicircularwave- 1 guide section 10 at a distance equal to 0.5462R from the centerthereof as in the case of the first embodiment, whereas asemi-cylindrical dielectric 4 is disposed upon the inner wall of thelower semicircular waveguide section 20. That is, the apparent radiusfor the TE mode of the lower semicircular waveguide section 20 which hasthe same radius with that of the upper semicircular waveguide section 10is made greater by disposing the semi-cylindrical dielectric 4 withinthe lower section 20 so that the phase velocities of the TE modespropagating through the waveguide sections 10 and 20 may be differentfrom each other. As for the TE mode propagating through the lowersemicircular waveguide section 20, the apparent radius becomes greater,but the apparent radius for the TE mode propagating through the uppersemicircular waveguide section 10 becomes greater because of thepresence of the dielectric 3. As a consequence, there is no differencein phase velocity between the TE modes propagating through the upper andlower semicircular waveguide sections 10 and 20.

As is clear from FIGS. 3(c) and 3(d), the same effects can be attainedwhen the magnetic materials are used instead of the dielectricmaterials.

The fourth and fifth embodiments of the present invention employingmagnetic material are shown in FIG. 7. The fourth embodiment shown inFIG. 7(a) is substantially similar in construction to the firstembodiment described with reference to FIGS. 1 and 2 except that asemi-cylindrical magnetic material 5 is disposed within the uppersemicircular waveguide section 10 with a radius smaller than that of thelower semicircular magnetic material 6 is disposed to contact the innerwall of the lower semicircular waveguide section 20 having the radiusequal to that of the upper waveguide section 10.

The mode of operation of the fourth and fifth embodiments describedabove with reference to FIGS. 7(a) and 7(b) is substantially similar tothat of the first and third embodiments described with reference toFIGS. 1 and 2 and FIG. 6(b), respectively.

The mode filter for the circular waveguide in accordance with thepresent invention described so far has various advantages hithertounattained by the prior art mode filters, but the problem of thereflected waves in the filter is left unsolved. However, the presentinvention can also solve this problem as will become more apparent fromthe following embodiments to be described in conjunction with FIGS. 8and 9.

FIG. 8 is a longitudinal sectional view of the waveguide shown inFIG. 1. The circular waveguide generally designated by l is divided intothe upper and lower semicircular waveguide sections 10 and 20 by themetallic slab 2, and the semi-cylindrical dielectric 3 is disposedwithin the middle section 13 with a radius smaller than the upperwaveguide section 11 or 15. When the effect of the reflected wavestraveling back and forth in the waveguide 1 is not negligible, resistorelements or films 2 and 2" are disposed upon the partition wall 2 in theinlet and outlet waveguide sections 11 and 15 in opposed relation withthe inner walls thereof. The TE modes, which propagate throughthe upperand lower semicircular waveguide sections 10 and and which are out ofphase at the outlets of the waveguide sections 10 and 20, may beabsorbed by the resistor element or film 2" so that the problem of thereflected waves may be overcomed. The resistor film 2' at the inlet canabsorb the still remaining reflected waves of the TE, and TE modes whichare out of phase in the upper and lower semicircular waveguide sections10 and 20. Thus, multireflection can be prevented. The resistor elements2 and 2" are disposed at right angles to the directions of the electricfield intensities of the TE modes which are in phase in the upper andlower semicircular waveguide sections 10 and 20 so that they are almostnot affected when the thickness of the resistor films or elements 2' and2" is made sufficiently smaller relative to the wavelength. Although thetwo resistor films or elements 2' and 2" have been described as beingdisposed at the inlet and outlet of the waveguide respectively, it issufficient in practice to place only the resistor element or film 2" atthe outlet of the waveguide. In FIG. 8 the upper and lower semicircularTE modes opposite in phase are subjected to asymmetrical modetransformation at the output section of the metallic partiation plate.Since all of the electric 'fields of these modes have a component inparallel with the metallic plate, the current becomes in parallel withthe thin resistance films when the latter are disposed in parallel withthe metallic partiation plate as shown in FIG. 8 so that theasymmetrical modes, which have been transformed in mode, may be absorbedover an extremely short distance. Whereas a few meters of a helixwaveguide is required to absorb the asymmetrical modes, according to thepresent invention only lpercent of the length of the helical waveguideis required.

FIG. 9(a) shows a rectangular thin metallic slab 2 having therectangular resistor plates or films 2' and 2". FIG. 9(b) shows themodification of the thin metallic partition slab 2 having the tapered orpentagonal resistor films or elements 2' and 2" which are more effectivein absorbing the reflected waves than the rectangular resistor elementsshown in FIG. 9(a).

In addition to the variations and modifications described so far,further variations and modifications occur to those skilled in the artwithout departing from the scope of the present invention. For example,for input and output waveguide sections 11 and 15, helical waveguides orring mode filters may be utilized in order to absorb the undesired modesgenerated by the imperfections of the waveguides. Furthermore, insteadof the semicircular waveguide sections, helix waveguides may beutilized.

So far only the absorption of only the TE mode has been taken intoconsideration, but the waveguide in accordance with the presentinvention may be so designed as to absorb other higher circular modes.In this case, it is of course necessary to change the positions of thedielectric or magnetic materials inserted into the waveguide because theelectric and magnetic field distributions of the higher modes aredifferent from those of the TE mode.

What is claimed is:

l. A circular waveguide mode filter of the type with low transmissionlosses for the desired TE, mode and with high attenuation for theundesired TE modes (where n a 2) comprising a circular waveguidecomprising a pair of semicircular waveguide sections having differentradii, and a dielectric so disposed in one of said pair of semicircularwaveguide sections with a smaller radius that the propagation of saidundesired TE modes may not be adversely affected by said dielectric.

2. A circular waveguide mode filter as set forth in claim 1 wherein saiddielectric is disposed at a position spaced apart from the center ofsaid one semicircular waveguide section with a smaller radius by adistance equal to 0.5462R where R the radius of said one semicircularwaveguide section.

3. A circular waveguide mode filter as set forth in claim 2, comprisinga thin metallic slab positioned to partition said pair of circularwaveguide sections, and a resistive material in contact with said thinmetallic slab.

4. A circular waveguide mode filter as set forth in claim 1 wherein saiddielectric is replaced by a magnetic material in said one semicircularwaveguide section with a smaller radius.

5. A circular waveguide mode filter as set forth in claim 4, comprisinga thin metallic slab positioned to partition said pair of circularwaveguide sections, and a resistive material in contact with said thinmetallic slab.

6. A circular waveguide mode filter as set forth in claim 4, comprisinga thin metallic slab positioned to partition said pair of circularwaveguide sections, and a resistive material in contact with said thinmetallic slab.

7. A circular waveguide mode filter as set forth in claim 1, comprisinga thin metallic slab positioned to partition said pair of circularwaveguide sections, and a resistive material in contact with said thinmetallic slab.

8. A circular waveguide mode filter of the type with low transmissionlosses for the desired TE mode and with high attenuation for theundesired TE modes 9. A circular waveguide mode filter as set forth inclaim 4 wherein v said first and second dielectrics are replaced by afirst and second magnetic materials in said pair of semicircularwaveguide sections, respectively.

10. A circular waveguide mode filter as set forth in claim 8, comprisinga thin metallic slab positioned to partition said pair of circularwaveguide sections, and a resistive material in contact with said thinmetallic slab.

I UNITED STATES PATENT OFFICE CERTIFICATE OF C0ERECTION Patent No. 8Dated January 22, 1974 sadakuni Shimada, etal Inventoflsabove-identified patent It is certified that error appears in the andthat said Letters Patent are hereby corrected as shown below:

Column 3, line 51 "illustrating" shouldbe' -'-i1lustrating change"claim-'4" to claiim 8-- Column 10," Claim 1-9, line 2 Signed and sealedthis 3rd day of December 197 (SEAL) Arrest:

McCOY M. GIBSON JR. c. MARSHALL PDANN Arresting Officer Commissioner ofPatents USCOMM-DC 6087 6-! F ORM PO-OSO (10-59)

1. A circular waveguide mode filter of the type with low transmissionlosses for the desired TE01 mode and with high attenuation for theundesired TE0n modes (where n > OR = 2) comprising a circular waveguidecomprising a pair of semicircular waveguide sections having differentradii, and a dielectric so disposed in one of said pair of semicircularwaveguide sections with a smaller radius that the propagation of saidundesired TE0n modes may not be adversely affected by said dielectric.2. A circular waveguide mode filter as set forth in claim 1 wherein saiddielectric is disposed at a position spaced apart from the center ofsaid oNe semicircular waveguide section with a smaller radius by adistance equal to 0.5462R where R the radius of said one semicircularwaveguide section.
 3. A circular waveguide mode filter as set forth inclaim 2, comprising a thin metallic slab positioned to partition saidpair of circular waveguide sections, and a resistive material in contactwith said thin metallic slab.
 4. A circular waveguide mode filter as setforth in claim 1 wherein said dielectric is replaced by a magneticmaterial in said one semicircular waveguide section with a smallerradius.
 5. A circular waveguide mode filter as set forth in claim 4,comprising a thin metallic slab positioned to partition said pair ofcircular waveguide sections, and a resistive material in contact withsaid thin metallic slab.
 6. A circular waveguide mode filter as setforth in claim 4, comprising a thin metallic slab positioned topartition said pair of circular waveguide sections, and a resistivematerial in contact with said thin metallic slab.
 7. A circularwaveguide mode filter as set forth in claim 1, comprising a thinmetallic slab positioned to partition said pair of circular waveguidesections, and a resistive material in contact with said thin metallicslab.
 8. A circular waveguide mode filter of the type with lowtransmission losses for the desired TE01 mode and with high attenuationfor the undesired TE0n modes (where n > or = 2) comprising a circularwaveguide comprising a pair of semicircular waveguide sections havingthe same radius, a first dielectric so disposed in one of said pair ofsemi-circular waveguide sections that the propagation therethrough ofsaid undesired TE0n modes may not be adversely affected; and a seconddielectric so disposed in the other semicircular waveguide section thatthe propagation of both of said TE01 and TE0n modes therethrough may beaffected by said second dielectric.
 9. A circular waveguide mode filteras set forth in claim 4 wherein said first and second dielectrics arereplaced by a first and second magnetic materials in said pair ofsemicircular waveguide sections, respectively.
 10. A circular waveguidemode filter as set forth in claim 8, comprising a thin metallic slabpositioned to partition said pair of circular waveguide sections, and aresistive material in contact with said thin metallic slab.